Herbicides have a phytotoxic effect on plants, and so they are used in plant production for controlling weeds or totally inhibiting their growth. Herbicides can be non-selective, which means that they destroy all growth, or selective in part or full, whereby they can be used for inhibiting the growth of weeds of only certain crop plants. Most herbicides used today are selective and they can be applied onto growing crop plants without damaging them. The problem with the use of selective herbicides is that particularly when a weed and a crop plant are of a closely related genus, the weed is usually resistant to herbicides. Further, the selectivity between crop plants and weeds changes with the use of herbicides. The increased use of herbicides has also been observed to make weeds more resistant to herbicides. Insufficient or impaired effect, in turn, has led to a further increase in the amount of herbicides used.
Herbicides are also an environmental problem. They are usually synthetic compounds that have a toxic or harmful effect not only on plants but also on animals. The herbicides used in agriculture decompose in the soil mainly by the effect of micro-organisms. The rate of microbiological decomposition of herbicides varies with the structure of the herbicide and the conditions of decomposition. The rate of decomposition can also depend on the amount of herbicide used in such a way that an increase in the amount slows down the decomposition. The herbicides used in agriculture are also spread to surface water and to other water systems, in which they decompose very slowly causing even more severe damage to the environment.
Despite the drawbacks, the use of herbicides has increased greatly over the years. Because of the environmental and other problems involved, efforts are made to keep the use of herbicides in check and under control through legislation.
The effect of herbicides can be improved by various additives or adjuvants. The best results are achieved with naturally susceptible species, but the effect can sometimes also be extended to resistant species. If the resistance is based, for example, on the thickness of the wax layer and/or weak migration of the herbicide in a plant, an adjuvant added to the mixture may impair the strength of the wax layer e.g. by improving the penetrativeness of herbicide molecules.
Adjuvants, however, are synthetic preparations, and they too are a hazard to the environment. Apart from intensifying the phytotoxic effect of herbicides, adjuvants are also phytotoxic as such. As a result of synergism, the harmful effects on both crop plants and the environment increase.